1. Technical Field
The present disclosure generally relates to a waveform generator, and more particularly, to an onion waveform generator and a spread spectrum clock generator (SSCG) using the same.
2. Description of Related Art
A spread spectrum clock generator (SSCG) gradually modulates a system clock to allow the working frequency to change within a specific range, so that the radiation energy produced by the system clock can be evenly distributed in a small segment of a spectrum and the formation of an energy peak at a single frequency can be avoided. Thereby, through a spread spectrum technique, electromagnetic radiation energy can be distributed so that the radiation of electromagnetic interference (EMI) can be reduced.
FIGS. 1A-1B illustrate output spectra obtained by using different modulation waveforms. Presently, delta-sigma (ΔΣ) modulation is mostly adopted in SSCGs. A characteristic of ΔΣ modulation is that the spread spectrum effect of ΔΣ modulation is not affected by process temperature or voltage. Referring to FIG. 1A, in the output spectrum obtained by using the sine waveform for spread spectrum modulation, a depressed power attenuation and accordingly two raised ends are produced, as shown in FIG. 1A. Namely, because the spectral energy is not evenly distributed on the extended frequencies, the maximum energy attenuation is not achieved and accordingly the spread spectrum effect is not so satisfactory.
Referring to FIG. 1B, in another conventional technique, a triangular waveform is used for ΔΣ modulation. The triangular waveform modulation technique is the most commonly used technique in existing SSCGs. However, as shown in FIG. 1B, in a SSCG adopting the triangular waveform modulation, the power at the middle of the extended frequency range is still slightly depressed. Thus, the raised ends still exists in the output spectrum, and the spread spectrum effect is still not so satisfactory.